Environmental problems on a global scale including warming temperature are being watched, concerning especially reduction of CO2 elimination, which is considered to have a major effect on the warming trend across the globe, setting procedure of regulatory criterion of CO2 has become the center of great interest.
In regard to CO2 reduction, it is one of major challenges to improve fuel efficiency of motorcars, for example, and it is strongly hoped to improve performance of sliding material and lubricant.
It is an function of the sliding material to exhibit excellent wear resistance and low friction coefficient for a sliding region in a heavy frictional wearing environment, and application of various hard film materials such as DLC is forwarded, recently.
In a case of coating hard DLC on a soft substrate such as aluminum alloy or so, at a time of producing the sliding member applied with the hard film of this kind, it is known to form an intermediate layer made of various metals and carbides in thickness on the order of nano-meter on the substrate surface in order to improve adhesiveness between substrate metal and DLC.
For example, with regard to the aluminum substrate, a method is proposed to form a hard film, such as DLC, after forming nitrogenous chromium film on the aluminum alloy substrate finished to maximum surface roughness of 3 μm or less (refer to patent documents 1).
With respect to ferrous substrate, it is proposed to form the top lop layer mainly containing DLC on the substrate through the four-layer structured intermediate layer composed of first layer made of metallic layer of Cr and/or Al, second layer made by mixing metal of Cr and/or Al, and one or more metal selected from W, Ta, Mo and Nb third layer made of at least one or more metal selected from W, Ta, Mo and Nb, and fourth layer made of amorphous material containing carbon and at least one or more metal selected from W, Ta, Mo and Nb, toward the top layer from the surface of the substrate (refer to patent documents 2).
On the other side, as a coating technique for the hard film with the intent of reducing friction, it is proposed to reduce the friction and improve anti-seizability, wear resistance under lubrication by forming hard film such as DLC on a steel substrate formed with undulation and micro-dimples by means of, for example, shot peening and fine grain peening in combination (refer to patent documents 3).
In this way, it is desired to establish a technique for improving adhesion of DLC with the substrate, especially low hardness substrate made of aluminum alloy, magnesium alloy or so, in order to apply DLC having excellent friction characteristics to various members practically.
Furthermore, most of sliding members in various types of machine equipment, including automotive engines are used under lubrication by industrial lubricants, therefore a technique is strongly required for enabling remarkable reduction of the friction, especially under the presence of lubricant as a technique for contributing to improvement of the global environment.    Patent Documents 1: JP, 2007-100133, A    Patent Documents 2: JP, 2003-171758, A    Patent Documents 3: JP, 2001-280494, A
In the method described in the aforementioned patent documents 1 and 2, adhesion strength of DLC film is improved by forming the intermediate layer, however it is not possible to obtain sufficient effects especially in a case of using soft substrate such as aluminum alloy by reason that the intermediate layer has a thickness as thin as a few μm. That is, the DLC film is apt to peel off or wear out easily according to deformation of the substrate under the frictional condition of high bearing pressure, and there is a problem in that the DLC film is not possible to be applied to the soft substrate.
On the other side, in the method described in the patent documents 3, the large undulation in excess of thickness of the film is formed by the shot peening or cutting work, accordingly drastic deterioration is caused in the adhesion and wear resistance in a case of coating the DLC film on the soft substrate such as aluminum alloy or so. Additionally, the DLC film is easy to peel off at the position formed on a sharply projecting part of the substrate, and there is also a problem in that it is scarcely possible to recognize the reduction effects of the friction even under lubrication as the result of disclosing underlying metal of the substrate.